The present invention generally relates to semiconductor device manufacturing, and more particularly to the formation of semiconductor-on-insulator (SOI) fins using porous silicon.
In SOI technology, the SOI layer and bulk substrate layer are separated by a continuous insulating layer referred to as a buried oxide (BOX). This separation or isolation of the device layer (i.e., the SOI layer) can result in significant benefits and performance improvements including, for example, less junction capacitance and leakage; greater resistance to ionizing radiation, electrical noise and heat; and immunity to CMOS latch-up.
Only a few methods to fabricate SOI substrates are proven to be commercially viable. In one, called BESOI (bond-and-etch-back SOI), two Si wafers are oxidized at the surface and the oxidized surfaces are bonded together and then one of the two bonded wafers is etched or thinned to provide a thin SOI device layer. In this method and its variations, the thickness of the buried oxide layer can be controlled during oxidation of the surfaces of the two silicon wafers. Therefore, the buried oxide can be made to have any desired thickness; however, the above process may introduce impurities at the bonded interface between the two oxidized surfaces. Further, etch-back or thinning of one silicon wafer to produce the thin SOI device layer may result in a non-uniform surface presenting challenges during subsequent processing.
In another well-known method, called SIMOX (separation by implantation of oxygen), a selected dose of oxygen ions is directly implanted into a Si wafer, and then the wafer is annealed in an oxygen ambient at a high temperature so that the implanted oxygen is converted into a continuous buried oxide layer. The thickness of the buried oxide layer in the SIMOX method is mostly dependent on the implanted oxygen dose and the thermal oxidation conditions. Moreover, in SIMOX, the Si over-layer is thinned to a desired thickness during the thermal oxidation, after which the surface oxide is stripped off.
Normally, SIMOX processes may require an oxygen concentration or dose of about 3E17 cm−2 to about 5E17 cm−2 to form a low-defect, continuous buried oxide layer that separates the Si over-layer from the substrate. In order to facilitate the implantation of this high level of oxygen ions in a reasonable period of time, high-current implanters are specifically built for SIMOX application at an extra cost.